Hello!

In some recent analysis I ran into a subtlety that caught me by surprise: Data.Map.mapKeysMonotonic has a misleading name.

A monotonic function is not a strictly increasing function, but merely non-decreasing. However, mapKeysMonotonic requires that it's mapping function be injective, which means it really only supports increasing functions.

valid (mapKeysMonotonic (\x -> if x `elem` [1,2] then 2 else x) (fromList [(1, "a"), (2, "b"), (3, "c")])) == False

The docs hint at this with "This means that @f@ maps distinct original keys to distinct resulting keys."

However, I'd propose that we deprecate this name and rename to something like mapKeysIncreasingor mapKeysAsc (to follow the pattern of other *Asc functions). We should also clarify the docs.

From https://github.com/haskell/containers/issues/617

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